1. Field of the Invention
The present invention relates to an automatic clutch for a vehicle such as an automobile and, more particularly, to a method of controlling the automatic clutch when the vehicle is to be moved.
2. Description of the Prior Art
In recent years, vehicle automatic transmissions have been used more widely so as to facilitate driving operation. Such a vehicle can use an automatic clutch in which a hydraulic actuator controlled by an electronic control unit drives a friction clutch (e.g., a dry-type, single plate clutch).
As a control method of the automatic clutch of the above-specified kind when starting to move such a vehicle, there have been proposed in the prior art a method, by which application of the clutch is changed in accordance with the rise in the r.p.m. (i.e., the number of revolutions per minute) of an engine (as is disclosed in Japanese Patent Laid-Open No. 50-12648, for example), and a method by which the applying speed of the clutch is changed depending upon the engine r.p.m. (as is disclosed in Japanese Patent Laid-Open No. 52-51117, for example).
In both of these control methods, however, the clutch applying operation is conducted after the engine r.p.m. has risen. This invites the following disadvantages: (i) not only the useful life of the clutch but also the mileage of the vehicle is lessened because the engine r.p.m. has already been increased to a considerably high value before the clutch is applied, while the vehicle itself is stopped, so that the engine is idly accelerated and the clutch slips to a large extent in its partially applied state; and (ii) the start responsiveness of the vehicle is seriously degraded because it takes a considerable period before the engine r.p.m. is recovered after an accelerator pedal has been depressed and because the clutch is controlled in accordance with the recovery. Since the vehicle is reluctant to start if the start responsiveness is bad, moreover, the accelerator pedal is further depressed which further adversely affects the vehicle by aggravating the first disadvantage.
In order to overcome these disadvantages, a start control method has been proposed in which the rate of change of the engine r.p.m. is detected to apply the clutch at a speed corresponding to the depression of the accelerator pedal, while it is positive to interrupt application of the clutch, when it becomes negative, and to again apply the clutch when a positive value is restored.
In the case of this control method, however, it is necessary to measure accurately the range of change of the engine r.p.m. This accurate measurement can be ensured, when fluctuations in the engine r.p.m. are small as in a gasoline engine, but is difficult when r.p.m. fluctuations are large as in a Diesel engine. As a result, delay in the timing to release the clutch will drop seriously the engine r.p.m. to cause an engine to stop or to knock. If the clutch is released prematurely, on the contrary, there arises another disadvantage in that the engine r.p.m. is accelerated while idling.